1. Field of the Invention
This invention relates to vapor generators and, more particularly, to an improved transition means for sealingly attaching a casing to a water-cooled membrane wall of the type employed in vapor generators used to produce steam in electric or industrial power plants.
Most modern vapor generators have water-cooled wall panels known as membrane walls. The membrane walls are composed of rows of vertically extended tubes, laterally spaced on centers wider than the tube diameter, which are connected by flat metal bars known as membranes. These bars are positioned 180.degree. apart on the outside surface along the length of the tubes and continuously welded thereto and to adjacent tubes so as to form a continuous wall surface comprising an alternate succession of elongate circular tube surfaces and elongated flat membrane surfaces that are stepped inwardly of the outermost surface of the tubes defining intervening spaces between the tubes. The welds may be formed by various known means and are usually formed on both sides of the membrane wall.
Metallic sheets or plates, known as casing, are attached to the membrane wall to form a gas-tight cased enclosure, for example, such as a windbox for housing the vapor generator's burners and for distribution of combustion air. It is essential that the casing be connected to the membrane wall in a gas-tight manner.
2. Description of the Prior Art
At present, connection of the casing and membrane wall is accomplished by the placement of short blocks of filler bar in the spaces between the tubes, adjacent the outer side of the membrane bars. An arrangement in which filler bars are welded in place between tubes is disclosed, for example, in U.S. Pat. No. 3,357,408. The filler bars are horizontally and vertically seal welded between adjacent tubes to provide a flush, continuous surface transversely across the tubes and the intervening spaces. The casing, in turn, is seal welded to the filler bars and tubes. This type of structural arrangement stiffens the tubes and restricts the ability of the tubes to expand and contract.
During operation of the vapor generator, considerable temperature differences between the membrane wall and the casing subject them to different amounts of thermal expansion. The stiffened arrangement, moveover, has been found to be quite sensitive to accelerated temperature excursions, such as are experienced in some cycling vapor generators during changes in the vapor generator's operating conditions, and may eventually lead to excessive thermal stresses and resultant tube failures.